Jin from BTS may have made his debut as a member of EXO if things didn't work out the way they did. Before joining BTS, he was offered an audition by an SM representative who spotted him on the streets. He did not attend because he thought the whole thing was a scam.
CL from 2NE1 was actually a JYP trainee before joining YG Entertainment. CL still maintains close relationships with JYP singers to this day.
Hyorin could have debuted as a 3 member group from JYP Entertainment. But during the preparations, things fell apart and she eventually joined Starship Entertainment and became a member of SISTAR.
B.A.P Zelo had ties with YG Entertainment, but was held back because of his young age. He finally took his chance as a trainee with TS Entertainment and debuted as a member of B.A.P.
At a young age, WINNER's Mino was affiliated with the pre-debut members of Block B but eventually joined YG Entertainment as a member of WINNER.
Zico was once affiliated with SM Entertainment and almost became a trainee. However, he found his success after joining Block B.
Hani was affiliated with JYP Entertainment but was eventually cut. She found her success as a member of EXID.
During the time legendary group H.O.T was popular, and when BoA and the Shinhwa members were all trainees, G-Dragon, who was very young at the time, had signed under SM Entertainment. He eventually left and joined YG Entertainment — debuting as a member of Big Bang.
ChoA had tried out for both SM and JYP before joining FNC Entertainment — eventually debuting as a member of AOA.
Hyunseung was a YG trainee preparing to debut with the members of Big Bang. He was cut but eventually found his success when he joined Cube Entertainment. He made his debut as a member of B2ST, making hit tracks like "Troublemaker" with his label mate HyunA.
Kahi was always in the shadows of SM Entertainment, most notably as a backup dancer for BoA. She eventually found her success with Pledis Entertainment — becoming the first member of After School.
Jun.K was accepted into both YG and JYP Entertainment but decided with the latter — and eventually debuted as a member of 2PM.
IU tried out for JYP Entertainment numerous times but was rejected every time. She was eventually signed under Loen Entertainment and made her debut as a solo artist at the age of 15.
4minute HyunA was a member of Wonder Girls early on, but had to step out due to health reasons. Once she got better, she joined Cube Entertainment and found her success as a member of 4minute.